1. Field of the Invention
The present invention relates to a photographic film cassette of which rotation of a spool causes photographic film to advance to the outside of a cassette shell. In particular, the invention is an improvement of the cassette shell which protects the film from damage.
2. DESCRIPTION OF THE RELATED ART
A known photographic film cassette includes a photographic filmstrip (hereinafter referred to as film) positioned so that the film leader does not protrude from the cassette shell prior to loading the cassette in a camera. Such a device is disclosed in U.S. Pat. No. 4,887,776. Simple film-advancing mechanisms of the camera are used with this type of cassette and include a construction which rotates a spool to unwind the film, thereby causing the leader to move through a film passageway and exit from the cassette.
In this construction, the roll of the film wound about the spool is prevented from loosening in order to transmit rotation of the spool to the leader. In such a cassette, the spool is constituted of a pair of spool pieces which are slidable relative to each other. A first spool piece is provided with a first flange, whereas the second spool piece is provided with a second flange. When one of the spool pieces is rotated, a cam mechanism associated with both spool pieces operates so as to shorten the interval between the flanges by sliding the spool pieces relative to one another. The first flange is provided with a circumferential lip which projects from around its peripheral edge for contact with a side of the outermost turn of the roll of film in order to prevent the roll from loosening. When sliding the spool pieces, the outermost turn of the roll is clamped between the second flange and the inside of the lip so that the roll is rotatable integrally with the spool and is thus prevented from loosening.
There is, however, a problem in this type of film cassette where the flanges are brought closer to one another to clamp the roll of film. A lateral edge of the film is contacted with, and rubbed against, the lip of the flanges with considerable force during the advancement of the film. Accordingly, the film edge may be worn by contact with the lip. This wearing gives rise to fine dust which may collect in the cassette. Also, the film edge is partially deformed or bowed in a successive manner by the interaction with the lip. The film edge, however, remains straight upstream from the lip. This causes an abrupt flexure of the film and, due to the rigidity of the film, a fracture may occur in the film edge due to the lateral deformation of the film in a sharp angular manner.